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Five belts of metamorphosed sedimentary and volcanic rocks underlie southwestern Maine and southeastern New Hampshire: Middle Ordovician Falmouth-Brunswick sequence; Middle and Late Ordovician Casco Bay Group, and Late Ordovician to Early Silurian rocks of the Merribuckfred Basin; Late Ordovician to Early Silurian rocks of the East Harpswell Group; Silurian to Early Devonian rocks of the Central Maine Basin; and highly tectonized enigmatic rocks of the Rye complex of uncertain age. Stratigraphic reassessment and new U/Pb zircon ages support a model of east-directed Middle Ordovician subduction beneath Miramichi, a peri-Gondwanan block, and formation of the Falmouth-Brunswick–Casco Bay volcanic arc complex that is roughly correlative with arc activity on strike in New Brunswick. Passive Late Ordovician sedimentation in a reducing restricted backarc basin followed. Late Ordovician to Early Silurian volcanic rocks and volcanogenic sediments (East Harpswell Group) support west-directed subduction under the Miramichi block. Late Ordovician to Early Silurian turbidites accumulated in the Merribuckfred Basin between the Falmouth-Brunswick–Casco Bay arc and Ganderia to the east. The collision of Ganderia with the Falmouth Brunswick arc in Late Silurian time represents an early phase of the Acadian orogeny, during which the Merribuckfred rocks were deformed, metamorphosed, intruded, and uplifted. Simultaneously and inboard, the Central Maine Basin received sediment eroded mostly from Laurentia. Later, during the Late Silurian and Early Devonian, uplifted Merribuckfred basin rocks became the major source of sediments for the Central Maine Basin. A later phase of the Acadian orogeny resulted in Middle Devonian deformation, metamorphism, and intrusion of rocks of all six belts.

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